The present disclosure relates to an image forming apparatus incorporating a sheet feed unit that is used to previously keep in stock a large number of sheets, such as paper sheets, to be supplied to a device.
Sheet feed cassettes are used, in image forming apparatuses exemplified by copiers and printers, for the feeding of cut paper sheets or the like. In a sheet feed cassette, a large number of unprinted sheets are kept in stock previously, and by a sheet feed unit provided near the sheet feed cassette, one sheet after another is separated and fed out from the topmost layer of the bunch of sheets stacked in the cassette.
The sheet feed unit is fitted with expendables such as a sheet feed roller and a pickup roller, and is thus configured to be easily mounted in and dismountable from the main body of the image forming apparatus on occasions of maintenance and replacement of those expendables.
Some sheet feed cassettes are provided with a sheet stacking plate on the top face of which sheets are stacked. The sheet stacking plate is supported, at its upstream-side end in the sheet feed direction, on the inner side of the bottom face of the cassette body, and is swingable, about this end as a pivot, at the downstream-side end in the sheet feed direction as a swinging end. The swinging end of the sheet stacking plate is raised up by a driving means such as a lift motor provided in the image forming apparatus. This permits the downstream-side end of the sheets stacked on the sheet stacking plate to move to a proper sheet feed position, enabling stable sheet feeding.
As a method for detecting the sheet feed position, it is common to read the output value of a sensor that detects the top face of the sheets or of the sheet stacking plate in a manner interlocked with the pickup roller arranged in the sheet feed unit. For example, a sheet feeding device is known which is provided with a first detection sensor for detecting whether or not a sheet has passed between a sheet feed roller pair and a second detection sensor (top-face detection sensor) for detecting the sheet feed position at which the sheet stacking plate is raised.